SEOUL, June 19 (Yonhap) -- Defense Minister Jeong Kyeong-doo called on military commanders Wednesday to tighten discipline and readiness amid criticism that a North Korean fishing boat drifted deep into South Korea waters and reached the shore undetected.

Concerns and criticism have mounted over possible loopholes in the military's maritime surveillance capabilities after the wooden boat carrying four North Koreans was found in waters quite close to a seawall of the port in the east coast city of Samcheok, Gangwon Province, on Saturday morning.

The site was around 130 kilometers south of the Northern Limit Line (NLL), the de facto maritime border between the two Koreas.

Calling the case "a very grave situation," Jeong demanded during a biannual meeting of top commanders that the military tighten operational and work discipline, and proactively push for measures to prevent recurrences, according to the defense ministry.

According to sources and witnesses, the boat was anchored at the dock, and civilians first spotted it with their naked eye and reported the case to local police.

Though the vessel was previously known to have been stranded due to an engine failure, it turned out to have had no such trouble.

Asked by South Korean fishermen where they were from, the North Koreans replied that they were from the North. One of them even asked South Koreans to borrow a cellphone.

The circumstances have sparked suspicions that they were not stranded but intentionally crossed the border for defection.

Of the four, two were returned to their home country Tuesday, according to their wishes. But the remaining two expressed their desire to defect to the South and have been going through due procedures required for such defectors, the unification ministry said.

It said that the boat was discarded at the consent of the captain.

In response to simmering criticism that the country's maritime border defense appears to have loosened, the Joint Chiefs of Staff (JCS) has said that its overall coastal and maritime defense operations had proceeded normally at that time, and detecting small wooden boats often has some limits depending on weather and other conditions.

"Surveillance systems usually spot vessels of iron or steel properties, and it is not easy to find slowly moving wooden objects at sea," a JCS officer said, adding the height of the waves on the day was nearly the same as that of the ship, making it difficult for the military to detect it.

Acknowledging that its radar operation system has "elements that need to be complemented," the military vowed to come up with measures, including the improvement of its radar and other surveillance systems, in order to prevent recurrences.

It was the second time in less than a week that a North Korean fishing boat has been rescued in the South.

Last Tuesday, six North Korean crew members on another fishing boat were found stranded south of the NLL in the East Sea and were sent back home on humanitarian grounds.

In Tuesday's case, the North asked the South via a naval hotline for help in rescuing the boat.